Electroluminescent display and method for driving the same

ABSTRACT

An electroluminescent display utilizing a capacitive display device. The display has an inherent minimum useful illumination term, during which it must be illuminated in order to be seen by an observer. The display is driven (actuated), so as to be illuminated, by pulses of actuating alternating current, and there will be more than one of these pulses during each said minimum useful illumination term. The pulse term, or duration, of the individual pulses is shorter than the pulse interval. The pulses may be shaped so as most efficiently to transfer energy to the display. The driving system improves the efficiency and lengthens the life of single displays, and permits higherintensity operation of multiple displays. A useful driving system includes a pulse transformer which produces the desired actuating current when it is itself pulsed.

United States Patent Webb 1 1 *Sept. 23,1975

[54] ELECTROLUMINESCENT DISPLAY AND 3,813,575 5/1974 Webb 315/167 METHODFOR DRIVING THE SAME [75] Inventor: Robert D. Webb, Hacienda Heights,Primary V. Ronnec Assistant ExaminerLawrence J. Dahl [73] Assignee:Sigmatron, Inc., Santa Barbara, At'omeyi Agent P Pastmiza Calif.

[*1 Notice: The portion of the term of this patent subsequent to May 28,1991, has been 57 ABSTRACT dlsclalmed An electroluminescent displayutilizing a capacitive [22] Filed. Oct 1 1973 display device. Thedisplay has an inherent minimum useful illumination term, during whichit must be illu' Pl No? 402,378 minated in order to be seen by anobserver. The dis- Reiated US Application Data play is driven(actuated). so as to be illuminated, by Continuation of Ser. No 96434'Nov 8' [971] pulses of actuating alternating current, and there will bemore than one of these pulses during each said minimum usefulillumination term The pulse term, or

1 I I H T duration, Of the individual PUISCS iS shorter than lhE [51]Int. Cl. H H658 37/00 pulse interval" pulses may be Shaped so as most ofSearch I D 4 I H 3 I R efficiently i0 transfer energy [0 the The driv-340/336; ing system improves the efficiency and lengthens the life ofsingle displays, and permits higher-intensity op- [56] References Citederation of multiple displays. A useful driving system includes a pulsetransformer which produces the de- UNITED STATES PATENTS sired actuatingcurrent when it is itself pulsed. 3,519,880 7/l970 Yoshiyarna et al. .13l5/l69 R X 3,560,784 2/l97l Steele et al. 313/92 3,618,071 11/1971Johnson 315/169 R X 18 Claims, 10 Drawings Figures 33 a -1 43 F 1n :550a 0 2 j l6 S6MNT 45:

SELECT ELECTRULUMINESCENT DISPLAY AND METHOD FOR DRIVING THE SAME Thisis a continuation of application Ser. No. 196.434. liled Nov. 8. l97l.now U.S. Pat. No. 3.8l3.575. issued May 28. I974.

This invention relates to electroluminescent display systems, and to amethod for driving the same.

Electroluminescent display panels of the capacitive type depend fortheir excitation and resulting luminos ity upon the application ofanalternating current across two electrodes between which there isdisposed a layer of electroluminescent material. Examples of this classof panel are those in which the layer is a compressed powder. a vacuumdeposited layer, or a ceramic layer commonly made from a bakedcompressed layer of electroluminescent material and glass frits. Anexample of a suitable electroluminescent panel will be found in US. Pat.No. 3.560.784, issued to Gordon N. Steele and Edwin J. Soxman on Feb. 2,l97l, entitled Dark Field, High Contrast Light Emitting Display".

It is presently common practice to illuminate electro luminescentdisplay by continuously applying an alternating current across the saidtwo electrodes for the full useful illumination term. There have beenother practices wherein an alternative current is applied in periodicbursts. but these bursts have been of substantial duration. Suchpractices have heretofore been believed to be necessary to driveelectroluminescent displays, and they have therefore been widely used.However, they do have unfortunate side effects. Among these side effectsare excessive heating of the panel which leads to its earlier breakdown.a neessary reduction of voltage in operation in order to minimize thiseffect. and a consequent reduction in peak output luminosity.Electroluminescent displays produce light most efficiently at relativelyhigh frequencies and voltages (amplitudes), but these are the sameparameters which lead to breakdown of the display when prior art drivingpractices are used. Accordingly, it has not heretofore been possible tooperate an electroluminescent panel of the capacitive type to bestadvantage utilizing commonly known driving techniques and circuitry.

Furthermore, when multiplex systems are used, i.e.. when a plurality ofdisplays are simultaneously driven by a common drive, the division ofcurrent from the single source has resulted in a lesser illuminationlevel. Attempts to correct this situation by raising the voltage andapplying the current according to the prior art practices lead to earlyfailure of the displays.

It is an object of this invention to provide an electroluminescentdisplay system and a method for driving the same wherein relativelyhigher voltages and frequencies may be utilized to drive a given displayso as to maximize the luminous output in multiplexed installations, andto lengthen the life and increase the efficiency in both single andmultiplex installations. Average current consumption is decreased, andthe system can operate with simpler and smaller power supplies.

The system and method according to this invention include anelectroluminescent display of the capacitive type connected to a sourceof alternating current. The display has an inherent minimum usefulillumination term, i.e.. a minimum period of time it must be illuminatedat or above some luminous level in order to be seen by an observer. Thedisplay is driven (activated), so as to be illuminated, by pulses ofactuating alternating current. The pulse interval between the start ofadjacent pulses is preferably shorter than the said minimum usefulillumination term. The pulse term. or per iod of duration of theindividual pulses. is shorter than the pulse interval so there willpreferably be more than one of said pulses per minimum usefulillumination term.

According to a preferred but optional feature of the invention. thepulses are shaped so as more efficiently to transfer energy to thedisplay. because their energy output is closely conformed to the energyacceptance and characteristics of the display.

According to still another preferred but optional feature of thisinvention. the number of cycles per pulse is restricted to that in whicha substantial increment increase in brightness results from each cycle.and preferably there will be about five or less per pulse, becausewithin this lesser number of cycles. each cycle provides a differentialincrement of brightness which is appreciably larger than that providedby later cycles.

The invention will be fully understood front the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a circuit diagram showing the presently preferred embodimentof the invention;

FIG. 2 is a fragmentary cross-section partly in schematic notation,taken at line 2-2 in FIG. 3;

FIG. 3 is a plan view of an illustrative example of a suitable displaypanel for use with the invention;

FIG. 4 is a diagram showing the preferred pulse train and wave form foruse in the practice of this invention together with a diagram of theresulting luminosity of the driven display;

FIGS. 5 and 6 are graphs showing peak voltage and peak current versuscxitation frequencies to produce a given luminosity. utilizing the waveform of FIG. 4;

FIGS. 7, 8, 9 and 10 are schematic graphs showing certain designconsiderations for the system.

In FIGS. 2 and 3 there is shown an electroluminescent panel I0 of typeuseful with this invention. FIG. 2 shows it as comprising a layer II ofelectroluminescent material disposed between a pair of electrodes l2,13, one of which will preferably be transparent. This will usually bethe common electrode. and is connected to lead 14. There may be, andusually there will be, additional layers and components. For one suchexample, reference may be had to the aforesaid US Pat. No. 3,560,784.

It is customary for numerical displays to be disposed in segments suchas segments I6, 17, I8, 19, 20, 21 and 22 (See FIG. 3). By means ofselecting various segments to be illuminated, any desired numeral fromzero through nine can be displayed in accordance with known techniques.Segment I6 is shown connected to lead 15. Similar leads are connected toeach individual one of the segments, and the segments are isolated fromone another so they can be illuminated individually. Ordinarily, therewill be more than one of these panels 10 to build up a larger display.If so, each will be provided with its own driving circuitry.

The driving circuitry includes a transformer 25 having a primary winding26, a secondary winding 27, and a core 28. The primary will have fewerwindings then secondary. Its output will ordinarily be about 200 voltsrms, and a convenient winding ratio is I0: 1. Very inexpensive audiotransformers may be used for this purpose. The core will be selectedsuch that a single pulse applied to the primary winding will produce anoutput alternating current pulse of the desired characteristics.

A transformer has the characteristic of ringing'l meaning that a singlepulse will create a train of sine waves of diminishing amplitude. Thecore is selected so as to limit the number of cycles having an amplitudeabove the minimum voltage needed for excitation (there is such aminimum, and voltages below this level are not considered part ofapulse"). These cycles will be few in number. for each pulse, and willdefinitely be less than would occupy a full pulse interval. With manypractical transformers, at the most three, and preferably only one orone and one-half, cycles will have a voltage sufficient to causeluminosity. A minimum of one cycle is needed for illumination. Theselection of the number of cycles per pulse will be discussed in greaterdetail below.

A source of voltage 30, such as a battery or a buss bar, is connected toone lead of the primary winding. The other lead of the primary windingis connected to ground 31 through transistor 32. The transistor may bean RCA 40327, the base of which is connected to a pulse source 33 whichis a digit drive device to illumi nate segments ofa given digit (orpanel). Pulses are periodically supplied to transistor 32. The selectionof which segments light up is otherwise made. The collector is connectedto the primary winding. and the emitter is connected to ground 31.

One lead of the secondary winding is connected to ground 35, and itsother lead is connected to electrode 12. This electrode is common andrelates to all of the segments. The electrodes comprising the segmentson the opposite face are connected to individual leads 15, 37, 38, 39,40, 41 and 42, each of which is connected to a respective switch means43a-43g. The switch means comprises transistors whose bases areconnected through respective resistors 45a-45g to segment select logic46, which will provide a pulse to turn on to flow selected ones of theseven switch means, and cause the respective selected segment orsegments to light up when a pulse is generated in the secondary winding.The switch means are all identical. Conveniently, they may comprise atransistor with its base connected as aforesaid, its collector connectedto the electrode of the respective one of the segments. and its emitterto ground 47.

in operation, a wave pulse, such as shown at 48, is applied across theprimary winding. This will generate a sinusoidal wave. such as shown at49 in FIG. 3. It is a ringing wave of decreasing amplitude, only thefirst few cycles, and preferably only the first one or one and one half,having sufficient amplitude to actuate the display. Because this wavegoes to a common electrode, potentially all of the segments could lightup. However, only those whose switch means will permit current flow willin fact light up. Accordingly, during the time a respective segment isto light up. a signal will be applied to its respective switch means topermit current flow therethrough. Then these selected segments willlight when excitation voltages are applied across the electrodes.

All previous displays of this type, utilizing frequencies in excess of afew hundred hertz, have recommended the usage of continuous sine orsquare wave excitation continuously for the full duration of theillumination term. Occasionally actuation has been suggested usingintermittent bursts of pulses of substantial duration. in these priorart arrangements, the exciting voltage was applied for a substantialnumber of cycles,

and then. especially in multiplexed applications, was repeated after apause. As a consequence of the contin uous drive during the period ofexcitation of each burst the voltage and illumination had to he helddown to prevent destruction of the panel. The upper frequency andvoltage limits under continuous excitation are determined by rates ofpower dissipation. as well as by the breakdown voltage limitation, whichmay vary as functions of display surface area. These previousarrangements still will function. but it has now been found that shortpulse. low percentage of illumination term excita tion as describedherein is more useful. especially for large area multiplex displayapplications.

A suitable transformer for this system will convert a dc power sourcevoltage to about 600 volts peak-topeak amplitude terminals. Each segmentof the display is individually selected by grounding it through thetransistor of its respective switch means. These transistors 43 shouldhave a breakdown voltage equal to the excitation voltage. However,reduced ratings merely cause incomplete turn-off of the segments. it hasbeen found that reduction of excitation voltage by as little as 25%reduces light emitted as a consequence of its actuation to a level belowthat which is visible under normal ambient conditions. In selecting theparameters of the transformer. it is only necessary that the cyclesafter the number desired for actuation. Of course, means can be providedto chop off all cycles other than the exact number desired.

The results of exerting actuation for less than the full illuminationterm. and in the manner described hereinafter. are shown in FlGS. 5 and6 wherein graphs 50 and 51 in solid line represent actuation at 12.5% ofthe illumination term, the long dashed lines 52 and 53 representactuation at 6.25% of the illumination term. short dashed lines 54 and55 represent actuation at 3.125% of the illumination term. and thedotted lines 56 and 57 represent actuation at 1.56% of the illuminationterm. These graphs illustrate the current and voltage relationships tofrequency for providing l5 foot- Lambert. time averaged brightness whichis produced by pulses of one and one-half full sine waves appliedperiodically to occupy the respective utilization of illumination term.In fact, the curves represent utilization of the wave train shown inFIG. 4, in which only the first three half cycles as illustrated aresufficient amplitude as to actuate the display. It will be seen in FIG.5 that, for the same luminosity, the peak voltage may be increased asthe percentage of illumination term occupied by pulses of actuatinglevel decreases. Accordingly, the device can be driven at higherfrequencies, where the efficiency in illuminating the display isgreatest, and still produce relatively less heat, thereby extending thelife of the display.

Similarly, the current is plotted in the same manner in FIG. 6, and itwill be noted that, as the percentage of the illumination term occupiedby excitation cycles decreases, the same luminosity may be obtained withincreased current flow but for materially lesser periods of time.thereby again decreasing the breakdown rate of the panel. it is the rmsvalue which determines the heating effect, and this situation isimproved with a frequency increase, while still enabling a highercurrent level to be used.

As a practical means of operation, this invention could utilizecontinuously-running sources of single or multiple pulses of suitableamplitude to illuminate a segment, passing some pulses and blockingothers, such as in the case of l 2.5% of the illumination term, passingevery eighth pulse and blocking the first seven.

The criteria for driving single and multiplex displays will now bedescribed in greater detail. In the case of single displays, such asshown in FIG. 1, the techniques will be used without reference to otherdisplays. In the case of multiplexed displays, there will be a pluralityof panels 10, and each will be actuated to illuminate the respectivedigit, but the actuation occurs serially and not simultaneously. Thus,each additional panel will be actuated by the same class of alternatingcurrent wave form for in the same manner. during each pulse interval. Itfollows that the pulse term times the number of panels (displays) willnot exceed the time duration of the illumination term. FIG. 1 thereforeillustrates the actuation of a single display, or the actuation of eachof a plurality of displays, in sequential operation.

FIGS. 4, 7, 8 and 9 illustrate certain chatacteristics of anelectroluminescent panel relative to currents which excite them toluminescence, and also present facts to be considered in selectingfrequency, voltage and wave form of the driving current.

The display, a capacitor, which will accept energy input during a finiteperiod, which period is determined by the specific physical andelectrical characteristics of the specific device. In particular theperiod during which it will accept energy is related to its timeconstant, which in turn is primarily a function of its internal seriesresistance and of its capacitance. The device will,

of course, accept energy only during the time current flows.

If the maximum efficiency of energy transfer is to be attained, then itis necessary to match the energyaccepting characteristics of the deviceto the output characteristics of the energy source. The quantity oflight emitted by the device is directly determined by the energyaccepted by the device from the power source.

Should the power source supply power in a form not acceptable by thepower device, either the device will not be optimally illuminated, orexcess power will be dumped into the system which will heat up thesystem and shorten its life. Thus, reaching a peak voltage or currentlimit for the system prior to the time the device can accept an optimumamount of energy results in a less-than-optimum amount of energy beingused for actuation. Should the voltage be increased to make the devicelight up to its optimum intensity under these conditions, the excessenergy will tend to destroy the device.

In FIGS. 7, 8 and 9, dashed lines 70, 71, 72 schematically illustratethe energy acceptance characteristic (ordinate) versus time (abscissa)of the electroluminescent capacitor. Solid lines 73, 74, 75 show threetypes of applied current wave forms with voltage as the ordinate andtime as the abscissa.

Line 73 is a very steep, flat sided nearly square wave, with a largeapplitude and short duration. Line 74 is a flat-topped square wavewhich, howeverm has sloping and curved sides which more nearly matchsegment 76 of line 74, and has a lesser amplitude and longer duration.Line 75 is a sine wave.

Attention is now called to the common area bounded by the upper level ofenergy acceptance of lines 70, 71 and 72, and the rising portions oflines 73, 74 and 75. These areas are shaded for illustration. It is thisarea which is proportional to the energy actually transferred to thedevic and which causes illumination. The area under the line showingapplied voltage (lines 70. 71 and 72) not also within lines 73, 74 and7S involves problems for the system.

Area 77 (FIG. 7) is quite small relative to area 78, and this class ofwave form is very disadvantageous. because with its high voltage peak itcan cause destructive current peaks during the time the device isaccepting energy. Area 79 in FIG. 8 showing the advantage of moreclosely tailoringthe shapes of the curves to correspond to one another.It also illustrates why a longer pulse term is useless. because thedevice will not accept further energy.

In FIG. 9 area 80 occupies nearly the same area as that under either ofthe curves. Optimum illumination will be caused. and the actuatingcurrent promptly drops off, while the illumination only graduallydiminishes. FIG. 9 shows. therefore, that only a short burst ofenergy.if properly tailored relative to the energy acceptance characteristics.will cause optimum illumination with minimum, stray energy to bedissipated. and minimum risk to system components. Thus, the wave formof the energizing current should be such as to supply energy to thedisplay at substantially the same rate it will be accepted by thedisplay.

Bearing the foregoing in mind, reference should now be made to FIG. 4.In order to fit this figure to any practical drawing scale. it has beennecessary greatly to foreshorten the abscissa. In point of fact, theperiod of time an electroluminescent device need be excited by appliedalternating current, in order to produce an op timum level ofillumination is only a small part of the illumination period, becausethe relaxation period during which the phosphor continues visibly toglow is much longer than the rise time, and its curve is much shallower.For example, in many electroluminescent devices, the rise period will beapproximately equal to the pulse term, and the relaxation period will beof the order of one millisecond regardless of the pulse term. It followsthat energy can be injected into an electroluminescent display muchfaster than it can be radiated as light.

Further, there are certain other considerations to be born in mind inconsidering FIG. 4 and the parameters of this system. The first is thatfor any display there is a maximum resulting light intensity, whateverthe details of excitation. In present systems, the applied voltage toreach this intensity is limited by the mode of operation and the heatwhich it generates. Another consideration is that the rate ofincrease inintensity oflight output during the rise period is much greater than therate of decrease in intensity during the relaxation per iod. Forexample, it will frequently be found that a 50% decrease in luminositymay take on the order of ten times the period of time it took to rise tomaximum intensity.

With the above in mind, it will be seen that successive charge anddischarge cycles will produce a smaller increase in intensity that thefirst, if identical pulses are applied before luminscense ceases. If thesecond and successive excitation cycles produce less of an increase inintensity than the first, it is apparent that efficiency will decrease.For example, in FIG. 4, there are shown three half-cycles 100, 101 and102. These result in increases in luminosity denoted by segments of theluminosity curve by segments 103, I04, 105 of the luminosity curve untilsegment 106 is reached, which is a point representing the maximum lightintensity which will be derived from the applied cycles of that pulse.When subsequent pulses are begun when the illumination level is abovezero, then the maximum light intensity may be somewhat higher. subject,of course, to the inherent limit of the display itself.

A study of the luminosity created by the equal actuating cycles willshow that increments A and B are sub stantially equal, and that linearresults are attained. while increment C is less than either of them.Accordingly. greater efficiency results if actuation is caused by alimited number of cycles per pulse. Fairly linear results are thenattained. FIGS. 7, 8 and 9 each illustrate a half cycle.

There is a very important consequence of the foregoing, namely that inallowing substantial relaxation of the phosphor after each period ofexcitation there would be expected to result a reduction of averageintensity. However. it has been found that by frequent excitation oflesser interval. less thermal dissipation need occur, and a higheramplitude of exciting current can be used. There results substantiallythe same average luminosity, and a higher operating efficiency withlower operating temperatures and an extended operating life.

Accordingly, as can be seen in FIG. 4, two sets of ac tuating cycles I10(including segments I00, I01 and 102) III are illustrated. The phosphoris premitted to return to an illumination level near or below that whichcannot be observed by the eye as being illuminated (even though it maybe excited, and have luminous intensity B at a level above extinction).In any event. it is at such a value that less than all of its excitingcycles produce significantly non-linear results. The three halfcycles ofcycle 11] will cause illumination increases denoted by segments 112,113, 114 until again in a maximum level 115 is reached. Although it isnot shown as such in schematic FIG. 4, level 115 will be somewhat higherthan point 105, because the actuation started while luminosity was notat zero, but instead at some higher value, still being less than thelower limit of visibility. Should further excitation occur after the fewcycles in a given pulse, more of its energy will simply have to bedissipated as heat. In FIG. 4, the exciting voltage has been showndirectly aligned with luminosity for convenience in disclosure of theeffect of each cycle, However, in actual result, there is a time lagbetween actuation and luminosity. This is easily observed by the use ofconventional instrumentation techniques.

Line 116 shows the level of luminosity below which an observer will notbe able to tell that the display is actuated in other words, even if itis excited, the flow is too dim. In the operation of this invention, itis possible to permit the luminosity to decay at zero before againpulsing the display, and if the luminosity produced by successive pulsesis sufficient, the eye will perceive an illuminated image. Depending onall the circumstances, this technique may be used. However, it may alsobe preferable not to permit the luminosity to decay all the way to zero,in which case the best efficiency it will be permitted to decay at leastto its limit of lowest visibility as exemplified by line I16. There islittle point in re-exciting the display while it is still glowingbrightly enough to be perceived.

Also, the term minimum useful illumination term" has been used herein. Asingle pulse as defined herein will not produce sufficient exicitationthat the display will be perceived by an observer. Persistcncy andacuity of vision are also involved. Therefore as a matter of definition,the basic module for actuation. namely that required at least toperceive the fact that it has occurred. has been defined. 01' course anypractical operation will involve durations longer than this minimum. butthe proportions defined with relation to the said minimum term alsoapply to the extended terms. which merely constitutes a sequence of manyof such minimum terms.

There are certain other considerations involved in the design of thepulse. especially as to the number of actuating cycles per pulse. FIG.10 shows as its ordinate the increment of luminosity caused per cycle ina pulse starting from zero luminosity. The abscissa represents the cyclenumber in each pulse.

Examination of FIG. I0 illustrates that for about the first five cycles,the increment of illumination. while decreasing. will remain reasonablyconstant, after which the value falls off fairly steeply to about twentycycles, and cycles beyond twenty have little. if any. of feet except, ofcourse, to heat up the display.

Therefore in the practice of this invention. one will use a number ofcycles per pulse wherein the pulse pro vides a substantial increment ofluminosity. i.e., below about twenty, and preferably in a substantiallylinear range. i.e., less than about five. FIG. 4 illustrates that, evenwithin this narrow limit, subsequent cycles provide a lesser incrementof luminosity.

This invention thereby provides a means for driving a phosphor with arelatively higher amplitude of exciting current than is tolerable inroutine operations and operating it at relatively shorter bursts andlesser percentage of the illumination term. Preferably, the cycles areapplied for less than about 25 percent of the illumination term inbursts of not more than three full cycles. The preferred range isbetween about one percent and about 25 percent of the illumination asthe total of the pulse terms.

As to the operation of the system of FIG. I, the segments to beilluminated are selected by segment select logic 46, which grounds therespective switches 430-431;. The digit drive is periodically pulsed soas to apply the wave forfn 49 to the drive. The wave form 49 will havenot more than three half-cycles of amplitude sufficient to causeluminosity above the limit of visibility. The ringing cycles beyondthose three will be of lesser amplitude. The digit drive is pulsed withthe frequency determined from the aforesaid considerations, and, shoulda pulse term of 10 percent of the illumina tion term be desired. forexample, the term of the indi vidual pulse will be divided into thetotal illumination term, and the frequency of the pulsation will be selected and applied at the digit drive. Accordingly, actuating currentsapplied in this manner will cause the selected segments to illuminateand remain illuminated so long as the exciting current is applied.

This invention is not to be limited by the embodiment shown in thedrawings and described in the description, which is given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

What is claimed is:

1. An electroluminescent display system comprising: a capacitiveelectroluminescent diaplay having a pair of electrodes and anelectroluminescent layer therebetween, the display having an inherentminimum useful illumination term for which term it must be actuated inorder to be observed as an illuminated surface. and during whichalternating current of voltage at least equal to a minimum actuatingvoltage necessary to cause visible illumination is applied thereto inorder to create a luminous surface; a source of such alternating currenthaving an output frequency such that the period of three of its cyclesis less than the said minimum illumination term; and means forperiodically and intermittently applying said alternating current tosaid electrodes in pulses, each pulse containing a number ofcontributing cycles of such amplitude as to contribute a significantincrement of increased luminosity. the pulses being applied with suchfrequency that said contributing cycles are applied for actuation for atotal time of less than about 25% of said minimum useful illuminationterm.

2. A display system according to claim 1 in which said means applies tocontributing cycles to the electrodes for a total period between about0. l "/r and about 25% of the said minimum useful illumination term.

3. A display system according to claim 1 in which the number of saidcontributing cycles in each pulse is less than about twenty.

4. A display system according to claim 1 in which the number ofcontributing cycles in each pulse is such that each contributing cyclecontributes a substantially equal increment of increase in luminosity.

5. A display system according to claim 4 in which the number of saidcontributing cycles in each pulse is less than about five.

6. A display system according to claim 3 in which said means appliessaid contributing cycles to the electrodes for a total period betweenabout 0.1% and about 25% of the said minimum useful illumination term.

7. A display system according to claim 4 in which said means appliessaid contributing cycles to the electrodes for a total period betweenabout 0.1% and about 25% of the said minimum useful illumination term.

8. A display system according to claim 5 in which said means appliessaid contributing cycles to the electrodes for a total period betweenabout 01% and about 25% of the said minimum useful illumination term.

9. A display system according to claim I in which the source ofalternating current is a pulse transformer.

10. A method of illuminating a capacitive electroluminescent displayhaving a pair of electrodes and an electroluminescent layertherebetween. said display having as an inherent property the actuationto vilible luminosity as a consequence of the application across theelectrodes of an alternating current having a voltage and frequencysufficient to produce the luminosity, said method comprising applyingsaid alternating current in pulses, said pulses each comprisingcontributing cycles in number such that each cycle contributes asignificant increment of luminosity. the frequency of supplying saidpulses being such that the said contributing cycles occupy less thanabout 25% of the inherent mini mum useful illumination term for whichthe display must be actuated in order to be observed as an illuminatedsurface.

11. A method according to claim 10 in which the said contributing cyclesare applied to the electrodes for between about 01% and about 25% ofsaid minimum useful illumination term.

12. A method according to claim 10 in which the number of saidcontributing cycles in each pulse is less than about twenty.

13. A method according to claim 10 in which the number of contributingcycles in each pulse is such that each contributing cycle contributes asubstantially equal increment of increase in luminosity.

14. A method according to claim 13 in which the number of contributingcycles in each pulse is less than about five.

15. A method according to claim 12 in which said means applies saidcontributing cycles to to the electrodes for a total period betweenabout 0.1% and about 25% of the said minimum useful illumination term.

16. A method according to claim 13 in which said means applies saidcontributing cycles to the electrodes for a total period between about0.1% and about 25% of the said minimum useful illumination term.

17. A method according to claim 14 in which said means applies saidcontributing cycles to the electrodes for a total period between about0.15% and about 25% of the said minimum useful illumination term.

18. A method according to claim 10 in which each pulse comprises a trainof ringing contributing cycles.

1. An electroluminescent display system comprising: a capacitiveelectroluminescent diaplay having a pair of electrodes and anelectroluminescent layer therebetween, the display having an inherentminimum useful illumination term for which term it must be actuated inorder to be observed as an illuminated surface, and during whichalternating current of voltage at least equal to a minimum actuatingvoltage necessary to cause visible illumination is applied thereto inorder to create a luminous surface; a source of such alternating currenthaving an output frequency such that the period of three of its cyclesis less than the said minimum illumination term; and means forperiodically and intermittently applying said alternating current tosaid electrodes in pulses, each pulse containing a number ofcontributing cycles of such amplitude as to contribute a significantincrement of increased luminosity, the pulses being applied with suchfrequency that said contributing cycles are applied for actuation for atotal time of less than about 25% of said minimum useful illuminationterm.
 2. A display system according to claim 1 in which said meansapplies to contributing cycles to the electrodes for a total periodbetween about 0.1% and about 25% of the said minimum useful illuminationterm.
 3. A display system according to claim 1 in which the number ofsaid contributing cycles in each pulse is less than about twenty.
 4. Adisplay system according to claim 1 in which the number of contributingcycles in each pulse is such that each contributing cycle contributes asubstantially equal increment of increase in luminosity.
 5. A displaysystem according to claim 4 in which the number of said contributingcycles in each pulse is less than about five.
 6. A display systemaccording to claim 3 in which said means applies said contributingcycles to the electrodes for a total period between about 0.1% and about25% of the said minimum useful illumination term.
 7. A display systemaccording to claim 4 in which said means applies said contributingcycles to the electrodes for a total period between about 0.1% and about25% of the said minimum useful illumination term.
 8. A display systemaccording to claim 5 in which said means applies said contributingcycles to the elEctrodes for a total period between about 0.1% and about25% of the said minimum useful illumination term.
 9. A display systemaccording to claim 1 in which the source of alternating current is apulse transformer.
 10. A method of illuminating a capacitiveelectroluminescent display having a pair of electrodes and anelectroluminescent layer therebetween, said display having as aninherent property the actuation to vilible luminosity as a consequenceof the application across the electrodes of an alternating currenthaving a voltage and frequency sufficient to produce the luminosity,said method comprising applying said alternating current in pulses, saidpulses each comprising contributing cycles in number such that eachcycle contributes a significant increment of luminosity, the frequencyof supplying said pulses being such that the said contributing cyclesoccupy less than about 25% of the inherent minimum useful illuminationterm for which the display must be actuated in order to be observed asan illuminated surface.
 11. A method according to claim 10 in which thesaid contributing cycles are applied to the electrodes for between about0.1% and about 25% of said minimum useful illumination term.
 12. Amethod according to claim 10 in which the number of said contributingcycles in each pulse is less than about twenty.
 13. A method accordingto claim 10 in which the number of contributing cycles in each pulse issuch that each contributing cycle contributes a substantially equalincrement of increase in luminosity.
 14. A method according to claim 13in which the number of contributing cycles in each pulse is less thanabout five.
 15. A method according to claim 12 in which said meansapplies said contributing cycles to to the electrodes for a total periodbetween about 0.1% and about 25% of the said minimum useful illuminationterm.
 16. A method according to claim 13 in which said means appliessaid contributing cycles to the electrodes for a total period betweenabout 0.1% and about 25% of the said minimum useful illumination term.17. A method according to claim 14 in which said means applies saidcontributing cycles to the electrodes for a total period between about0.15% and about 25% of the said minimum useful illumination term.
 18. Amethod according to claim 10 in which each pulse comprises a train ofringing contributing cycles.